1. Field of the Invention
The present invention relates to a method used in a wireless communication system and related communication device, and more particularly, to a method of handling transmission and reception involving an almost blank subframes (ABS) subframe in a wireless communication system and related communication device.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3GPP Rel-8 standard and/or the 3GPP Rel-9 standard are developed by the 3rd Generation Partnership Project (3GPP) as a successor of a universal mobile telecommunications system (UMTS), for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides a high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple UEs, and communicates with a core network including a mobility management entity (MME), a serving gateway, etc., for Non Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques, such as carrier aggregation (CA), coordinated multipoint transmission/reception (CoMP), UL multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Furthermore, to mitigate load in a hot spot of a cell (e.g. a macro cell) caused by a large number of clustered UEs, a smaller cell (e.g. a micro cell or a pico cell) is usually deployed at the hot spot of the cell. In this situation, a transmission of the cell and a transmission of the smaller cell may interfere with each other, when transmitters of both the cells transmit signals to respective UEs at the same time. Therefore, interference mitigation is needed to mitigate such interference. For example, a set of subframes is “blanked” by a cell in order to protect resources of other cell(s) in order to mitigate interference to transmissions on the set of subframes from the other cell(s). The set of subframes blanked is called Almost Blank Subframes (ABS). A subframe allocated to a UE can be configured as an ABS subframe for realizing the interference mitigation. When the subframe allocated to the UE is configured as the ABS subframe, the eNB may not transmit a packet in the subframe such that eNBs of neighboring cells can serve UEs of the neighboring cells (e.g. picocells) without interference in the subframe. In this situation, the UE should not try to receive the packet in the subframe to reduce power consumption, since the UE will receive nothing. However, the eNB may need to transmit the packet a subframe even though the subframe is configured as the ABS subframe in certain situations.
For example, when a subframe is allocated to a UE in a downlink (DL) semi-persistent scheduling (SPS) and the subframe is also configured as an ABS subframe, it is unknown whether the UE should attempt to receive a packet in the subframe. If the UE attempts to receive the packet in the subframe and the network does not transmit the packet, the UE receives nothing and transmits a negatively acknowledgement (NACK) corresponding to the packet. However, the NACK is unnecessary, and the UE wastes power on transmitting the NACK.
On the other hand, it may happen that a UE transmits a packet on an uplink (UL) subframe, and attempts to receive a hybrid automatic repeat request (HARQ) feedback (e.g. an ACK or a NACK) corresponding to the packet on a DL subframe. However, the DL subframe is configured as an ABS subframe. It is not known whether the network should transmits the HARQ feedback and whether the UE should attempt to receive the HARQ feedback.
Furthermore, when a UE performs a random access procedure, messages corresponding to the random access procedure are exchanged between the UE and an eNB. For example, please refer to FIG. 1, which is a transmission sequence diagram of the random access procedure according to the prior art. According to FIG. 1, the UE transmits a random access preamble (i.e., message 1) to the eNB to initiate the random access procedure. After the eNB receives the random access preamble, the eNB replies a random access response (i.e., message 2) to the UE to confirm the reception of the random access preamble. Then, the UE transmits a scheduled transmission (i.e., message 3) to the eNB according to information included in the random access response. The eNB resolves contention if multiple UEs request for resource at the same time, and transmits contention resolution (i.e., message 4) to the UE for allocating the resource to the UE. It may happen that at least one of the messages is scheduled in an ABS subframe. In this situation, it is not known whether the eNB and the UE should transmit or receive the at least one of the messages.
Therefore, how to resolve transmission and reception involving an ABS subframe is a topic to be addressed.